The present invention relates generally to X-ray tubes, and, more specifically, to improved cooling of the targets therein.
An X-ray tube includes cathode and anode assemblies suitably mounted in an evacuated glass frame or housing. The anode assembly includes a target in the form of a disk which is rotated at high speed adjacent to a cathode which emits an electron beam against a focal track adjacent to a perimeter of the target. A small portion of the electrons are converted at the focal track as an X-ray beam which passes through a window in the housing for use in conventional manners.
In an X-ray tube, less than about 1% of the electrical energy is converted into X-rays, with the remainder of the energy producing waste heat in the target. Accordingly, dissipating the heat from the target is one of the most important functions of the X-ray tube and its housing. The X-ray tube is typically immersed in a cooling fluid such as oil which is channeled over the outside of the tube for removing the heat therefrom during operation. However, the heat generated at the target inside the tube housing must also be suitably dissipated therefrom.
The X-ray tube is typically operated in cycles having one period in which X-rays are generated followed in turn by a cooling period to limit the temperature of the various components of the tube for maintaining an acceptable life. In the first few minutes of the cooling period, cooling of the target is radiation dominated, with radiation heat transfer being proportional to the fourth power of temperature. After the initial radiation cooling period, heat transfer is dominated by conduction from the target and through the remainder of the anode assembly to the tube housing.
Since the target rotates during operation, it is mounted in suitable ball or journal bearings inside the housing which themselves have corresponding temperature limits of operation for ensuring a useful life thereof. The target is typically bolted to a rotor supported by the bearings, with the bolts also having corresponding temperature limits for effective useful life thereof. Accordingly, conduction of heat from the target necessarily heats the target bolts and supporting bearings, with the heating thereof being suitably limited for obtaining a suitable useful life.
The temperature limits of the target bolts and bearings therefore control the X-ray producing period and the cooling period in the operating cycle of the X-ray tube. It is desirable to maximize the X-ray producing period and minimize the cooling period so that the X-ray tube may be operated for longer periods. In a typical application where the X-ray tube is used in a Computer Tomography (CT) scanner, reduced cooling periods will correspondingly increase the number of CT scans in a given time improving the efficiency of operation of the CT scanner.
Although it is generally desirable to increase heat conductivity from the target to the supporting bearings, such conductivity must also be limited in the region of the target mounting bolts to prevent their overheating. A typical target is removably mounted to one end of a target shaft creating a joint thereat, with the target shaft in turn being mounted to the bearing rotor creating another joint thereat. Both joints are simple contact joints which inherently provide resistance to heat conduction thereat which is typically used for limiting the temperature of the attachment bolts at the two joints for effecting a useful life thereof. Accordingly, heat conduction through the joints is reduced, which reduces heat transfer into the bearings and in turn from the tube housing. This controls the respective durations of the X-ray producing and cooling periods of the X-ray tube operating cycle.